Sterilisation is an act or process that destroys or eliminates microscopic forms of life, e.g. micro-organisms, bacteria, etc. During the process of plasma sterilisation, active agents are produced. These active agents may include high intensity ultraviolet photons and free radicals, which are atoms or assemblies of atoms with chemically unpaired electrons. An attractive feature of plasma sterilisation is that it is possible to achieve sterilisation at relatively low temperatures, such as body temperature. Plasma sterilisation also has the benefit that it is safe to the operator and the patient. In the case of hand disinfection, the cool plasma may be used instead of alcohol gel, the repeated use of which can cause a number of skin related problems.
Low temperature atmospheric pressure plasmas may be used to replace conventional sterilisation methods and offer clear advantage over existing means of sterilisation in terms of their non-toxic nature, instant treatment effects, and the ability to produce the plasma at a range of energy levels and in a range of different forms. In a room temperature environment, plasma is usually supported by electro-magnetic (EM) fields. Electrons absorb energy from an electric field and transfer part of this energy to heavy particles in the plasma. If electrons are not given sufficient opportunity to transfer their energy, heavier plasma components remain at much lower temperatures than the electrons. Such plasmas are called non-thermal plasma and their gas temperatures can be as low as room temperature.
A non-thermal plasma can be used to create highly reactive plasma particles (including e.g. electrons, ions, radicals, and other chemically active species) and ultraviolet (UV) radiation, which in turn may be used to disinfect and sterilise biological tissue, external work surfaces or surgical instruments. For example, UV photons in the plasma may affect bacteria cells by inducing the formation of thymine dimers in their DNA. This inhibits the ability of the bacteria to replicate properly. This effect may be particularly useful where it is desirable to reduce the level of bacteria, but not totally destroy it, i.e. so as not to destroy the body's natural flora.
The closer the plasma source is located with respect to the biological tissue (or other surfaces) and the higher the electric field in the plasma, the higher the intensity and efficacy of the non-thermal plasma sterilisation treatment process.
WO 2009/060213 discloses a sterilisation system having a controllable (e.g. capable of modulation in an adjustable manner) non-ionising microwave radiation source for providing microwave energy for combining with a gas (e.g. an inert gas or a mixture of inert gases) to produce atmospheric plasma. One example of the system described therein included a power splitting unit arranged to split microwave energy (e.g. from a microwave feed structure such as a co-axial cable) between a plurality of plasma generating regions, wherein a gas feed was connected to deliver gas to each plasma generating region, and in which the outlets of the plurality of plasma generating regions were spatially arranged to deliver a substantially uniform blanket or line of plasma from a plurality of plasmas generated in each respective plasma generating region. It was contemplated to provide ten or more plasma generating regions housed in a frame defining an aperture, wherein the plasmas from the plasma generating regions were directed inwards from the frame to provide a blanket of plasma for items passed through the frame. In particular, this application described an apparatus for sterilising hands in which movable plasma jets were provided in a box in which the hands could be inserted.
To strike plasma it is desirable to have a high electric field (e.g. high voltage or high impedance condition). Accordingly, it is necessary to set-up a high impedance state in order to enable the high voltage (high electric field) necessary to break down the gas to be generated. In one embodiment discussed in WO 2009/060213, the high voltage (high impedance) condition is set up using a flyback circuit that uses a low frequency (e.g. radiofrequency) oscillator circuit and a transformer whose primary winding is connected to the low voltage oscillator circuit by a suitable driver and switching device (e.g. gate drive chip and a power MOSFET or BJT). The arrangement generates high voltage pulses or spikes which strike or otherwise initiate the plasma.
After the plasma is struck, the impedance seen by the microwave power feed structure changes due to the change of the non-conducting gas into the conducting plasma. Here it is desirable to efficiently deliver the microwave energy into the plasma in order to sustain it. It is desirable for all (or most of) the microwave energy to be coupled into the plasma. Accordingly, it is desirable to match the generator impedance (i.e. the impedance of the microwave power feed structure) to the impedance of the plasma.